The present invention relates to a toolholder device for machine tools and, more particularly, to a toolholder device capable of detecting overload thrust applied to the tool, such as a drill or tap, for preventing damage of the tool.
As is well known, when the discharge of chip is not carried out sufficiently during a cutting operation such as a drilling operation, the cutting torque increases to such a degree as to cause a damage to the cutting tool due to twisting. Furthermore, when a worn drill is fed forward at a constant rate, an excessive thrust is produced, causing a buckling of the drill.
In order to detect such excessive cutting torque and thrust during the cutting operation, an overload detection system for a machine tool has been proposed in U.S. Pat. No. 5,155,473.
In this proposed overload detection system, a drive toolholder or holder body driven by the drive spindle has a driven toolholder supported by the holder body in a manner to be rotatable and axially shiftable relative to the holder body. The driven toolholder is urged forwardly by a thrust setting spring. The drive holder body and the driven toolholder have their respective displacement detecting elements or teeth disposed circumferentially thereof in interdigitating relationship. Circumferentially adjacent displacement detecting teeth of the drive side and the driven side are brought into abutment with each other by means of a torsion coil spring connecting the driven toolholder with the drive holder body.
These displacement detecting teeth of the drive and driven sides have mutually engaging beveled surfaces at their mutually abutting sides. The beveled surfaces make a relatively small angle to the axial direction of the toolholder. When an overload axial thrust is exerted upon the tool and hence the driven toolholder, the displacement detecting teeth of the toolholder acts axially rearwardly on the displacement detecting teeth of the drive holder body at their beveled surfaces so that the detecting teeth of the driven toolholder are caused to shift circumferentially by virtue of the beveled surfaces. Thus, the axially rearward displacement of the detecting teeth is translated or converted into a circumferential displacement, which produces a relative circumferential displacement between the displacement detecting teeth of the drive and driver sides. The circumferential displacement thus obtained is used for detection of an axially rearward displacement of the tool and toolholder.
In the overload detection system described above, an axial displacement of the tool and toolholder is converted into a circumferential displacement by means of the beveled surfaces. This gives rise to a problem in that the amount of axial displacement of the toolholder influences on the depth of a hole to be cut by the tool. When the axial thrust force is such that its amount exceeds a preset value of the thrust load setting spring but is not so great as to cause a circumferential displacement sufficient to produce a detectable angular phase difference, the overload thrust will not be detected and the cutting operation will continue in normal way to finish a hole drilling. In this case, the drilled hole will have a smaller depth than a regular depth and the obtained depth is sometimes outside the tolerance limits.